This invention relates generally to the field of optical communications systems. More particularly, the invention relates to interface devices such as interleavers and deinterleavers that are used for interfacing between portions of Dense Wave Division Multiplexed (DWDM) systems that operate at channel spacings differing by a factor of two, between, for example, portions operating at 50 GHz per channel and portions operating at 100 GHz per channel.
As DWDM optical communications technology has progressed, the channel spacing has changed over a number of years from 200 GHz to 100 GHz to 50 GHz per channel. When a communications system is in the process of being upgraded from say 100 GHz per channel to 50 GHz per channel, it may be expedient to retain some older equipment in the system, for example, older equipment that was designed for use at 100 GHz per channel. The older equipment can be retained in the upgraded system by using interleavers and deinterleavers to interface between the older equipment and the newer equipment.
An interleaver combines an optical signal containing even channels with an optical signal containing odd channels. A 50 GHz interleaver, for example, combines an optical signal containing a set of even channels having 100 GHz spacing with an optical signal containing a set of odd channels having 100 GHz spacing and produces an output optical signal containing the set of even channels and the set of odd channels with 50 GHz spacing per channel.
A deinterleaver reverses the process of the interleaver. A 50 GHz deinterleaver, for example, separates even channels from odd channels to produce two output signals, an output signal containing the set of even channels having 100 GHz channel spacing and an output signal containing odd channels and having 100 GHz channel spacing.
The general principle of the interleaver is based on the interference of two light beams. The interference creates a periodic repeating output as different integral multiples of wavelengths pass through the device and the desired channel spacing of the interleaver is set by controlling the fringe pattern. Manufacturers today use fused-fiber Mach-Zehnder interferometers, liquid crystals, birefringent crystals, Gires-Tournois interferometers (GTI) and other devices to build interleavers and deinterleavers.
Of these, the GTI based interleaver and deinterleaver have many advantages over the rest. For example, a GTI based interleaver has very low insertion loss, has uniform response over a wide range of wavelengths (flat-top spectrum), and has minimal polarization dependence effect.
Chromatic dispersion must be considered for 10 Gbit/s and next generation 40 Gbit/s systems. Chromatic dispersion requirements for the higher bit rate systems are extremely tight. While there are currently many technologies being pursued for use in interleaver products, the dispersion performance will probably be a critical factor in determining which technology will be successful. To be successful, the interleaver must not only have a low dispersion value at the center ITU wavelength, but over the full useful passband of the device (i.e. the dispersion should not reduce the usable passband). Unfortunately, the GTI based interleaver has a very large dispersion of up to 70-200 ps/nm for a 50 GHz interleaver and up to 250-800 ps/nm for a 25 GHz interleaver.
U.S. Pat. No. 6,169,604, entitled xe2x80x9cNONLINEAR INTERFEROMETER FOR FIBER OPTIC DENSE WAVELENGTH DIVISION MULTIPLEXER UTILIZING A PHASE BIAS ELEMENT TO SEPARATE WAVELENGTHS IN AN OPTICAL SIGNALxe2x80x9d, issued to Cao, discloses a multiplexer that includes two non-linear interferometers, wherein each nonlinear interferometer (NLI) is a GTI with an internal xcex/4 wave-plate and an external xcex/8 wave-plate (FIGS. 8 and 9 in U.S. Pat. No. 6,169,604).
In co-pending patent application Ser. No. 09/874,925, entitled xe2x80x9cOPTICAL SIGNAL INTERLEAVER AND DEINTERLEAVER DEVICES WITH CHROMATIC DISPERSION COMPENSATIONxe2x80x9d, incorporated herein by reference, the present inventor discloses a dispersion compensated interleaver and a dispersion compensated deinterleaver, each of which includes an NLI and a GTI dispersion compensator.
In an NLI, the angular alignment of the c-axis of the wave-plates relative to the direction of polarization is both critical and difficult. The correct angular alignment of the direction of the c-axis relative to the direction of polarization of the light beam entering the wave-plate is 45xc2x0 for both the xcex/4 wave-plate and the xcex/8 wave-plate. Misalignment of as little as 1xc2x0, that is to say rotation of a wave-plate by as little as 1xc2x0 around the direction of the beam from 45xc2x0 to 46xc2x0 or to 44xc2x0, causes serious distortion of the spectrum shape and group delay of the interleaver (or deinterleaver). Some examples of the distortions arising from misalignment of the xcex/8 plate will now be presented.
Misalignment degrades the isolation between channels. For example, a misalignment of the xcex/8 wave-plate by 1xc2x0 reduces the isolation between even channels and adjacent odd channels by 10%.
Misalignment distorts the group delay. FIG. 1 shows the group delay of a perfectly aligned 50/100 GHz NLI based interleaver (or deinterleaver). FIG. 1 applies to both odd and even channels. FIGS. 2 and 3 show group delay for odd and even channels, respectively, for the same NLI based device as in FIG. 1, but with misalignment of the c-axis of the xcex/8 wave-plate of 1xc2x0, from 45xc2x0 to 46xc2x0. The distortion of the group delay in FIGS. 2 and 3 as compared to FIG. 1 is apparent. Note that the distortion of the group delay is different for odd channels than for even channels. Such serious asymmetric distortions of the group delay make it very difficult to compensate for chromatic dispersion.
It is an object of the present invention to provide an interferometer which separates or combines signals for odd and even channels (for use in an interleaver or deinterleaver) and which has a group delay characteristic that is free of distortion arising from misalignment of phase shifting components relative to the direction of polarization of light entering the interferometer.
It is an object of the present invention to provide an optical interleaver and deinterleaver in which the group delay is not subject to distortion arising from angular misalignment of optical phase shifter components around an axis defined by the direction of the optical beam.
It is a further object of the present invention to provide an interleaver and deinterleaver in which the isolation between channels is not subject to degradation arising from angular misalignment of optical phase shifter components around an axis defined by the direction of the optical beam.
It is a further object of the present invention to provide an interleaver and deinterleaver in which a GTI is used as a dispersion compensator, and in which the absence of angular misalignment allows more effective chromatic dispersion compensation.
The objects and advantages of the present invention are provided by an interferometer that is a combination of a Gires-Tournois interferometer with Faraday rotators (hereafter GTIFR). The GTIFR is intended for use in a deinterleaver or deinterleaver.
The GTIFR in accordance with the present invention is a Gires-Tournois interferometer with a 45-degree Faraday rotator between the mirrors of the Gires-Tournois interferometer and a 22.5-degree Faraday rotator in the light path of light entering and leaving the Gires-Tournois interferometer. The Faraday rotators are preferably garnets, though any other type of Faraday rotator may be used.
The structure of the GTIFR includes a Gires-Tournois interferometer that has a partially reflective mirror optically coupled to a highly reflective mirror. The mirrors are parallel and separated by a fixed distance d. In the cavity between the mirrors there is a 45-degree Faraday rotator in the light path. The partially reflective mirror provides a port for light to enter and leave the Gires-Tournois interferometer. Outside the Gires-Tournois interferometer, in the path of light entering and leaving the Gires-Tournois interferometer, there is a 22.5-degree Faraday rotator.
In the operation of the GTIFR, as used in the single GTIFR interleaver or deinterleaver of the present invention, plane polarized light containing signals for odd and even channels passes through the 22.5 degree Faraday rotator and becomes circularly polarized with 22.5 degree phase difference. The light is then reflected from the Gires-Tournois interferometer with a phase change that is a function of the GTI and of the 45 degree Faraday rotator. The light then passes through the 22.5 degree Faraday rotator for a second time, where the phase characteristic is again changed.
An interleaver in accordance with the present invention requires only one GTIFR. Likewise a deinterleaver in accordance with the present invention requires only one GTIFR.
A GTIFR in accordance with the present invention, when used in a deinterleaver in accordance with the present invention, receives optical signals that are plane polarized in one direction and that contain signals for a set of even channels and for a set of odd channel. The signals are reflected by the GTIFR and then enter a polarization beam splitter. A signal containing one set of channels is reflected from the polarization beam splitter and a signal containing the other set of channels is transmitted by the polarization beam splitter.
A GTIFR in accordance with the present invention, when used in an interleaver in accordance with the present invention, receives plane polarized optical signals for a set of even channels and for a set of odd channels, the direction of polarization for the even channels being perpendicular to the direction of polarization of the odd channels. The signals are reflected from the GTIFR and enter a polarization beam splitter where both sets of channels are transmitted by the beam splitter.
As disclosed in co-pending application Ser. No. 09/874,925, a Gires-Tournois interferometer can be used to compensate for chromatic dispersion in a deinterleaver or interleaver.
In a dispersion compensated deinterleaver in accordance with the present invention, the even channel and odd channel signals pass through a Gires-Tournois interferometer dispersion compensator before being reflected by the GTIFR.
In a dispersion compensated interleaver in accordance with the present invention, the even channel and odd channel signals pass through a Gires-Tournois interferometer dispersion compensator after being reflected by the GTIFR.